This invention relates to waste heat extraction by forced convection boiling, to means for preventing vapor bubbles at heating surfaces, and to organic Rankine cycles for electric power generation from waste heat.
Chip Cooling:
Waste heat from digital signal processors (DSPs) and other high heat flux electronic components (collectively “CPUs” herein) degrades performance. The task of extracting waste heat from CPUs consumes inordinate energy and is a major expense at data centers.
The CPU's surface area is small and the heat flux through that small surface area must be high to get rid of the heat. Adding fins to increase the surface area, and blowing air at the fins, cannot overcome the basic limitation of direct air cooling, which is its low heat flux. The diffuse distribution of molecules in air means that only a few molecules at a time can be in contact with a fin or other solid surface, so the heat transfer into the air is small, less than 1 W/cm2 (on the surface area of the fins). If forcing is employed, to blow more molecules toward the surface, residence time of air molecules at the surface for heat transfer will be short. Theory predicts, and practical experience is confirming, that direct air cooling of CPUs will soon become extinct as high heat flux is increasingly demanded. Even with fins and forcing, the maximum chip heat flux with air cooling is less than 100 W/cm2. Indirect liquid cooling is the preferred technology, having a chip heat flux up to 400 W/cm2. See “High Powered Chip Cooling-Air and Beyond” by Michael J. Ellsworth, Jr. and Robert E. Simons, Electronics Cooling (August 2005) http://www.electronics-cooling.com/articles/2005/.
Forced convection immersion liquid cooling (without change of state from liquid to vapor) has a much greater heat flux than any air mode. The heat flux achievable on a fin with forced convection immersion is 50 W/cm2 (square centimeters of fin area) which is fifty times more than jet impingement air cooling. Forced convection is a significant improvement over free convection immersion (where only buoyancy drives fluid flow) whose heat flux is only 1 W/cm2—approximately the same as air jet cooling. Examples of forced convection liquid cooling are Roy, U.S. Pat. No. 7,055,531 (Jun. 6, 2006), a motor-driven centrifugal impeller disposed in a chamber above a CPU surface; Remsburg, U.S. Pat. No. 6,604,572 (May 16, 2000), which features thermosiphon convection in a chamber above the CPU, without mechanical pumping means; Wu et al., U.S. Pat. No. 6,894,899 (May 17, 2005), a motor-driven centrifugal impeller; Burward-Hoy, U.S. Pat. No. 5,442,102 (Aug. 15, 1995), a motor-driven centrifugal impeller; and Farrow, et al. U.S. Pat. No. 6,945,314 (Sep. 20, 2005), which is also a motor-driven centrifugal impeller. Perfluorinated coolants (also known as FC coolants), which are highly inert, are safe to use for direct liquid cooling of electronic components. Change of state for waste heat extraction can be done by pool boiling or forced convection boiling. Pool boiling has a heat flux limit of only 20 W/cm2 which is inferior to forced convection immersion (50 W/cm2). The problem is that vapor from the heated fin or other surface blocks heat flux into the liquid, and becomes superheated so it is harder to condense. Vapor nucleates at the heated surface and small bubbles coalesce there until the buoyancy of the aggregated bubble is enough to detach it from the surface. The absence of forcing means to sweep vapor off the heated surface as soon as it forms means that the vapor becomes an insulating pocket of superheated gas impeding the heat flux into the liquid coolant. Superheat must be extracted at the condensing end, which is a waste of cooling power. Examples of pool boiling for CPU cooling are Paterson, U.S. Pat. No. 5,390,077 (Feb. 14, 1995), a button-like clamp-on coolant tank having fins and an internal baffle directing vapor flow up and radially out along internal fins; and Searls, et al. U.S. Pat. No. 6,550,531 (Apr. 22, 2003), wherein vapor rises at the periphery of a chamber, condenses with fin air cooling, then drips at the center of a CPU.
A special class of pool boiling devices is the heat pipe, a hermetic vapor cycle in a tube, wherein a working fluid such as water evaporates at an evaporating end and the vapor rises in the pipe to a condensing end, where it condenses and discharges its latent heat to an external heat sink, such as ambient air or other cooling means. The heat pipe has no moving parts. To draw condensate back down to the evaporating end to complete the vapor cycle, wicking or capillary means are disposed at the inner wall of the heat pipe. The pressure within the heat pipe is approximately the vapor pressure at the desired operating temperature of the CPU. An example of a heat pipe for cooling a CPU is Tsai et al., U.S. Pat. No. 7,352,580 (Apr. 1, 2008), featuring a battery of slanted heat pipes, with finned tube heat rejection for the condensing end; and Herring et al., U.S. Pat. No. 7,352,579 (Apr. 1, 2008) with a heat pipe spring-loaded onto the CPU for improved conduction. Heat exchange between the vapor and the returning condensate should be avoided so that there may be heat flux from the hot chip to the heat rejection means as directly as possible. Heat pipe thermal conductivity is four times higher than copper.
Forced convection boiling is the best of all known cooling modes, having a heat flux over 100 W/cm2. Again, the surface for this value is the heated surface, including fins, and not the chip surface. Vapor is continuously swept from the heated surface, which can be the interior of an indirect liquid cooling apparatus, and vapor is quickly replaced by the denser liquid, so heat transfer into the liquid is never impeded by an insulating bubble layer of superheated vapor. Forcing requires some forcing means, conventionally an agitator or a pump powered by an external motor, which advects the condensate.
Burward-Hoy, U.S. Pat. No. 5,441,102 (Aug. 15, 1995) teaches a motor-driven centrifugal pump disposed in a short chamber above the CPU as a replacement for the wicking capillary means of a heat pipe. Fluid flow in the Burward-Hoy device is down at the center of the evaporating end, over the center of the CPU, and then radially outward to the periphery, driven by the externally-driven centrifugal pump in the chamber, and then up to external heat rejection means. This is the opposite of the direction in a conventional heat pipe, where vapor rises in the center. Although Burward-Hoy mentions a heat pipe, what is described is a forced convection liquid cooling device employing no change of state. No condensing means are mentioned, and there appears to be no space for vapor to evolve. The backward fluid flow in Burward-Hoy would have vapor flowing up the periphery instead of up the center of the evaporating end of the heat pipe. Farrow et al. U.S. Pat. No. 6,945,314 (Sep. 20, 2005) also teach internal motor-driven pumping means for advecting fluid downward against the center of the CPU and then radially outward over the heated surface, like Burward-Hoy.
The disadvantage of this down-and-out flow in Burward-Hoy and Farrow et al. is that vapor, which would tend to form at the hottest part of the heated surface, over the center of the CPU, may become trapped in an insulating pocket over the hottest part of the CPU until buoyancy makes it rise up through the downward flow of liquid, engaging in heat exchange with the cooled fluid instead of the external heat rejection means of the heat pipe. Like Burward-Hoy, Farrow et al. do not mention condensation or any vapor cycle.
The Rankine Cycle:
The ideal Rankine cycle is a closed system (no mass flow in or out) in which thermal energy (heat) is input from the environment to a boiler, causing evaporation of a working fluid such as water. The vapor exports work to the environment, losing enthalpy by flowing through a turbine, and the remaining energy in the vapor at the turbine exhaust is rejected to the environment by a condenser. Work is imported from the environment to pump condensate from the condenser back into the boiler, renewing the mass flow cycle. In practice, there is a small amount of mass flow into the cycle in the form of make-up water because of losses at the condenser. FIG. 9 of the drawings shows a flow diagram of the conventional Rankine cycle.
The Organic Rankine Cycle:
Water is the usual working fluid for the Rankine cycle, in steam turbines for electric power generation, but the organic Rankine cycle uses an organic compound, such as a haloalkane, instead of water. The high molecular mass of organic compounds, compared to water, gives the organic molecules higher momentum at a given vapor temperature. The organic working fluid chosen should be environmentally benign, such as HFC-245fa, marketed by Honeywell as Genetron 245fa. At 25° C., saturated vapor of water has a density 287 times smaller than saturated vapor of HFC-245fa, so a compact Rankine cycle operating at low heat input temperature can be accomplished by using an appropriate organic working fluid. Advantages of organic Rankine cycle devices include (1) high cycle efficiency; (2) very high turbine efficiency (up to 85 percent); (3) low mechanical stress on the turbine; and (4) low rpm of the turbine allowing for direct drive of the electric generator without gear reduction. Power generation from low temperature waste heat is possible using an organic Rankine cycle, whereas a conventional steam cycle requires an input temperature very much higher, from combustion of fossil fuels or a nuclear heat source, in order to have satisfactory thermodynamic efficiency.
The Hermetic Rankine Cycle:
A totally hermetic Rankine cycle would not export or import energy in the form of work, nor would there be any mass flow in or out of the cycle during operation. Energy imported in the form of heat would flow through the device and be rejected at the condenser to the environment, as in a conventional Rankine cycle. An at least partially hermetic Rankine cycle, which conserves work within the cycle to power the pump, is an object of the present invention. Using an organic working fluid in the hermetic Rankine cycle would allow for an especially efficient forced convection waste heat extractor powered by heat flow alone. It is an object of the present invention to provide such an improved heat sink, and to provide means for extracting work from waste heat without any input of work from the environment.